Coding Standards: Remove unnecessary isset() check in get_calendar().

`$newrow` is defined right before the `for` loop, and the loop only toggles its value between `true` and `false`.

Follow-up to [34463].

Props justlevine.
See #63268.

git-svn-id: https://develop.svn.wordpress.org/trunk@60331 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/general-template.php

@@ -2493,7 +2493,7 @@ function get_calendar( $args = array() ) {
 	$daysinmonth = (int) gmdate( 't', $unixmonth );
 
 	for ( $day = 1; $day <= $daysinmonth; ++$day ) {
-		if ( isset( $newrow ) && $newrow ) {
+		if ( $newrow ) {
 			$calendar_output .= "\n\t</tr>\n\t<tr>\n\t\t";
 		}